1. Field of the Invention
The general field of the invention relates to the drilling and completion of wellbores in geological formations, primarily in the oil and gas industries.
Commercially available progressing cavity mud motors are used in many drilling applications. The particular field of the invention relates to a new type of long-lasting mud motor that is not based upon the typical progressing cavity design, but may be used in many similar or analogous applications.
2. Description of the Related Art
Typical rotary drilling systems may be used to drill oil and gas wells. Here, a surface rig rotates the drill pipe attached to the rotary drill bit at depth. Mud pressure down the drill pipe circulates through the bit and carries chips to the surface via annular mud flow. Alternatively, a mud motor may be placed at the end of a drill pipe, which uses the power from the mud flowing downhole to rotate a drill bit. Mud pressure still carries chips to the surface, often via annular mud flow.
Typical mud motors as presently used by the oil and gas industry are based upon a progressing cavity design, typically having a rubber type stator and a steel rotor. These are positive displacement devices that are hydraulically efficient at converting the power available from the mud flow into rotational energy of the drill bit. These devices convert that energy by having an intrinsically asymmetric rotor within the stator cavity—so that following pressurization with mud, a torque develops making the rotor spin. These devices also generally have tight tolerance requirements.
In practice, mud motors tend to wear out relatively rapidly, requiring replacement that involves tripping the drill string to replace the mud motor. Tripping to replace a mud motor is a very expensive process. In addition, there are problems using these mud motors at higher temperatures. It is probably fair to say, that if the existing mud motors were much more long-lasting, that these would be used much more frequently in the industry. This is so in part because the rotary steering type directional drilling controls function well with mud motors, providing relatively short radii of curvature as compared to standard rotary drilling long with drill pipes. Mud motors also work well with industry-standard LWD/MWD data acquisition systems.
As an alternative to using mud motors, there are turbine drilling systems available today. These are not positive displacement type motors. They work at relatively high RPM to achieve hydraulic efficiency, often require a gear box to reduce the rotational speed of any attached rotary drill bit, are expensive to manufacture, and are relatively fragile devices having multiple turbine blades within their interiors.
So, until now, there are two widely used basic alternatives—rotary drilling and the use of mud motors. The mud motors “almost work well enough” to satisfy many industry requirements. However, looking at the progressing cavity design a little more closely also reveals that the rotor must be asymmetric in its stator to develop torque. In general, positive displacement motors suffer from this disadvantage—they are generally not cylindrically symmetric about a rotational axis. This in turn results in requiring that the output of a shaft of the mud motor couple to a “wiggle rod” to decouple the unwanted motion from the rotary drill bit. Such eccentric motion results in unwanted vibrations in adjacent equipment—such as in directional drilling systems.